This research is aimed at an analysis of the functional roles of tropomyosin in the structure and organization of microfilament bundles in normal and transformed animal cultured cells. We found that: (1)\of five forms of tropomyosin found in "normal" rat cell lines, three are major tropomyosins and two are relatively minor tropomyosins; (2)\upon transformation, the level of one or both of the major tropomyosins is decreased and the levels of minor tropomyosins are greatly increased; (3)\the degree of these changes in tropomyosin patterns correlates well with the extent of morphological transformation; (4)\the changes in tropomyosin patterns were also observed in temperature shift experiments with rat-1 cells transformed with a ts Rous sarcoma virus mutant; (5)\the changes in the pattern of tropomyosin expression are regulated at the level of mRNA ratherthan by post-translational modification; and (6)\changes in tropomyosin patterns are consistently expressed with a variety of transformation agents including virus, chemical carcinogen, UV-irradiation, and DNA transfection. These results imply that this differential expression of tropomyosin between "normal" and transformed cells may be involved in the changes in actin cables associated with morphological transformation. To explore this possibility, we have purified each isoform of tropomyosins from rat cultured cells and will examine in vitro physiological properties of microfilaments with different sets of tropomyosin by various biochemical and biophysical methods. In vivo functional roles of tropomyosin will be determined by microinjection of different forms of tropomyosin or monoclonal antibodies into living cells. The long-term goal of these studies is to understand how differential expression of tropomyosin effects cell morphology changes upon oncogenic transformation and how actin cables rearrange in this process. (L)